This invention relates generally to inflatable restraint systems and, more particularly, to an apparatus and method for inflating an inflatable device such as an inflatable vehicle occupant restraint for use in such systems.
It is well known to protect a vehicle occupant using a cushion or bag, e.g., an "airbag cushion," that is inflated or expanded with gas when the vehicle encounters sudden deceleration, such as in a collision. In such systems, the airbag cushion is normally housed in an uninflated and folded condition to minimize space requirements. Upon actuation of the system, the cushion begins to be inflated, in a matter of no more than a few milliseconds, with gas produced or supplied by a device commonly referred to as "an inflator."
Many types of inflator devices have been disclosed in the art for the inflating of one or more inflatable restraint system airbag cushions. Prior art inflator devices include compressed stored gas inflators, pyrotechnic inflators and hybrid inflators. Unfortunately, each of these types of inflator devices has been subject to certain disadvantages such as greater than desired weight and space requirements, production of undesired or non-preferred combustion products in greater than desired amounts, and production or emission of gases at a greater than desired temperature, for example.
In view of these and other related or similar problems and shortcomings of prior inflator devices, a new type of inflator, called a "fluid fueled inflator," has been developed. Such inflators are the subject of commonly assigned Smith et al., U.S. Pat. No. 5,470,104, issued Nov. 28, 1995; Rink, U.S. Pat. No. 5,494,312, issued Feb. 27, 1996; and Rink et al., U.S. Pat. No. 5,531,473, issued Jul. 2, 1996, the disclosures of which are fully incorporated herein by reference.
Such inflator devices typically utilize a fuel material in the form of a fluid, e.g., in the form of a gas, liquid, finely divided solid, or one or more combinations thereof, in the formation of an inflation gas for an airbag. In one such inflator device, the fluid fuel material is burned to produce gas which contacts a quantity of stored pressurized gas to produce inflation gas for use in inflating a respective inflatable device.
While such an inflator can successfully overcome, at least in part, some of the problems commonly associated with the above-identified prior types of inflator devices, there is a continuing need and demand for further improvements in safety, simplicity, effectiveness, economy and reliability in the apparatus and techniques used for inflating an inflatable device such as an airbag cushion.
To that end, the above-identified Rink, U.S. Pat. No. 5,669,629 discloses a new type of inflator wherein a gas source material undergoes decomposition to form decomposition products including at least one gaseous decomposition product used to inflate an inflatable device.
Such an inflator can be helpful in one or more of the following respects: reduction or minimization of concerns regarding the handling of content materials; production of relatively low temperature, non-harmful inflation gases; reduction or minimization of size and space requirements and avoidance or minimization of the risks or dangers of the gas producing or forming materials undergoing degradation (thermal or otherwise) over time as the inflator awaits activation.
Inflator devices commonly employ an initiator device which, upon actuation, serves to ignite or fire a pyrotechnic formulation for the subsequent generation, formation, release or the like of inflation gas for the inflation of an associated inflatable device, for example. Pyrotechnic formulations normally constitute both a fuel and an oxidant. In normal pyrotechnic formulations, all the oxidant is usually supplied as a constituent of the formulation.
"Equivalence ratio" (.phi.) is an expression commonly used in reference to combustion and combustion-related processes. Equivalence ratio is defined as the ratio of the actual fuel to oxidant ratio (F/O).sub.A divided by the stoichiometric fuel to oxidant ratio (F/O).sub.S : EQU .phi.=(F/O).sub.A /(F/O).sub.S (1)
(A stoichiometric reaction is a unique reaction defined as one in which all the reactants are consumed and converted to products in their most stable form. For example, in the combustion of a hydrocarbon fuel with oxygen, a stoichiometric reaction is one in which the reactants are entirely consumed and converted to products entirely constituting carbon dioxide (CO.sub.2) and water vapor (H.sub.2 O). Conversely, a reaction involving identical reactants is not stoichiometric if any carbon monoxide (CO) is present in the products because CO may react with O.sub.2 to form CO.sub.2, which is considered a more stable product than CO.)
For given temperature and pressure conditions, fuel and oxidant mixtures are flammable over only a specific range of equivalence ratios. Mixtures with an equivalence ratio of less than 0.25 are herein considered nonflammable, with the associated reaction being a decomposition reaction or, more specifically, a dissociative reaction, as opposed to a combustion reaction.
Reactant mixtures and combinations having equivalence ratios of less than one are generally designated as "fuel-lean" formulations; those reactant mixtures and combinations having equivalence ratios equal to one are generally designated as "stoichiometric" mixtures; and those reactant mixtures and combinations having equivalence ratios of greater than one are generally designated as "fuel-rich" formulations.
In practice, pyrotechnic formulations for initiators are often designed to be over-oxidized to better ensure complete combustion of the fuel. For example, typical initiators containing a pyrotechnic formulation of zirconium potassium perchlorate (commonly referred to as "ZPP") have a formulation equivalence ratio in a range of 0.7 to 1.0. Generally speaking, initiator formulations having an equivalence ratio of greater than one have been sought to be avoided as such formulations generally contain insufficient oxidant to fully oxidize the available fuel and thus such fuel will not contribute to the sought reaction.
There is a continuing need and demand for further improvements in safety, simplicity, effectiveness, economy and reliability in the apparatus and techniques used for inflating an inflatable device such as an airbag cushion. More specifically, there is a need and a demand for an inflator device which can provide at least some of the benefits provided by the inflator of the above-identified Rink, U.S. Pat. No. 5,669,629, wherein a gas source material undergoes decompositional or dissociative-type reaction to form products including at least one gaseous product used to inflate an inflatable device while also providing more complete and repeatable inflation gas formation and thereby reduce or minimize the variability in performance between similar inflator devices under the same or similar operating conditions.